This invention relates combustible aerosol compositions, and in particular to combustible aerosol compositions that can be used for applications such as fire suppression.
Flame suppressants can be classified as either active (chemical) or passive (physical) suppressants. Active suppression agents react chemically with and destroy free radicals in the flame. Free radicals are very short-lived species that catalyze flame reactions, and their chemical removal or modification in turn suppresses the flame. Passive suppressants often seek to deprive the combusting fuel from oxygen by physically interfering from its transport to or access to the flame combusting fuel.
One form of active suppressant is a class of materials sold as Halon™, which are composed of brominated or chlorinated fluorocarbon compounds, e.g., bromochlorodifluoromethane (CF2BrCl) and trifluorobromomethane (CF3Br). These and competitive materials using similar chemistry have been used effectively as fire suppression agents for years, typically to protect electrical equipment since there is very little residue to clean up. These fire suppression agents typically interrupt the chemical reaction that takes place when fuels burn and depend on a combination of chemical effectiveness, e.g., quenching of free radicals, and some physical effectiveness, e.g., cooling the combustion flame and dilution of the combustion ingredients. Certain halogen-containing fire suppression agents, however, such as CF3Br, contribute to the destruction of stratospheric ozone. Although the materials are essentially nontoxic, passage through a flame or over hot surfaces can produces toxic fluorine compounds.
To reduce the environmental effects associated with halogenated fluorocarbons, many commercially available fire suppression agents designed today are passive, i.e., physically acting, agents. A passive suppressant does not react chemically with the flame. These fire suppression agents either blanket the burning material to deprive it of oxygen, or they dilute the oxygen in the environment to below the point that can sustain the flame, or they cool the burning surface below its ignition temperature. Examples of physically-acting fire suppression agents include sodium bicarbonate and sand as well as inert gases, e.g., carbon dioxide (CO2), water vapor (H2O), and nitrogen (N2). When applied to a fire, inert gases physically displace oxygen from the combustion region while simultaneously serving as a heat sink to reduce the temperature of the flame. The combination of the two physical actions results in suppression of the fire. Gaseous passive agents cannot be used as total flooding agents in occupied spaces because they must reduce the oxygen content below the amount that will sustain life. This is especially true for carbon dioxide because it also interferes with human respiration in addition to simple localized dilution of oxygen.
Physically-acting fire suppression agents are subject to certain issues and problems that can reduce their effectiveness at fire suppression. They typically a require large quantity of a physically-acting fire suppressant in order to suppress a fire and, consequently, equipment and storage must be large to accommodate the large quantity. Such large equipment is a disadvantage in limited spaces. Another disadvantage of physical suppressants is that they must often be applied directly to a combusting surface, which can inhibit their effectiveness against fires that are concealed or relatively inaccessible.
An alternative to the above suppressant agent systems is the use of a pyrotechnically-generated aerosol flame free radical suppressant. This generation method may provide particles of free radical suppressant materials of such small particle size that their free-fall velocity is less than the velocity of air currents in an enclosed space. As such, the particles stay suspended, and seek out even concealed fires such as those that might be found inside enclosed spaces such as aircraft cargo subcontainers (e.g., an LD-3 container used on commercial aircraft). The smoke-like suspension characteristics of the aerosol provide long “hang times,” referring to the length of time a single generator function can continue to suppress recurrent flame. Another benefit of such pyrochemically generated aerosol is that their ozone-depleting potential may approach zero, that their inhalation toxicity may be much lower than that of inert gas, and that no toxic irritant gases may be generated on passage through flame or with hot surfaces.
Unfortunately, existing combustible fire suppressant aerosols also experience a number of issues that can limit their effectiveness. For example, some combustible aerosol compositions have a limited operating temperature range of about 15.5° C. to about 35° C., and can fail to ignite at temperatures outside this range, or a product that ignites at higher temperatures will not ignite at lower temperatures, or a product that ignites at lower temperatures may not ignite or may combust too aggressively at higher temperatures. However, environments in which fire suppression systems are deployed can be subject to a much wider range of temperatures, such as from about −40° C. to about 71° C., thus limiting the effectiveness of combustible aerosol fire suppressants for many applications.
Prior attempts have suggested to cool the aerosol stream through the addition of solid carbonate or dicarboxylic acid salt coolants such as magnesium carbonate or magnesium oxalate in the combustion composition. However, these salts have very high decomposition temperatures, such as magnesium carbonate having a decomposition temperature of greater than 538° C. (1000° F.) and therefore acts only as an inert diluent below that temperature. Other prior attempts have suggested to cool the aerosol stream through the inclusion of hydrated magnesium oxalate or ettringite. However, these compounds release water and moisture at relatively low temperatures and will not allow the flame front to propagate resulting in poor ignition or again act as a diluent. For example, ettringite decomposes at less than 70° C. and about one third of the decomposition product is liquid water, and provides no cooling or temperature modulation benefits at many combustion temperatures across a wider range from as low as about 260° C. (500° F.) where magnesium carbonate is ineffective to 538° C. (1000° F.), which includes much of the temperature range where combustion processes occur while.